köken: From the present participle of währen (“to last, persist”), with rebracketing of währendes [noun], währender [noun], währendem [noun] as während des [noun], während der [noun], während dem [noun].
Etymology tree Proto-Indo-European *gʰe? Proto-Indo-European *ǵʰō Proto-Balto-Slavic *źō Proto-Slavic *za Old Polish za Proto-Indo-European *swé Proto-Balto-Slavic *sen Proto-Slavic *sę Old Polish się univ. Old Polish zasię Polish zasięclip. Polish zaś Clipping of zasię. First attested in the 16th century. Compare Czech zase and Kashubian zôs.